Method of constructing a dielectric ladder



June 27, 1967 M. H. FINK METHOD OF CONSTRUCTING A DIELECTRIC LADDER 2Sheets-Sheet 1 Original Filed Aug. 19 1963 INVENTO'R. 7W4J PM UnitedStates Patent 3,328,223 METHOD or CONSTRUCTING A DIELECTRIC LADDERMiller H. Fink, Centralia, Mo., assignor to A. B. Chance.

This is a division of application Ser. No. 303,728, filed Aug. 19, 1963,now U. S. Patent No. 3,225,862, which in turn is a continuation ofapplication Ser. No. 132,665, filed Aug. 21, 1961, now abandoned.

This invention relates to a dielectric ladder and the method of makingsame, and more particularly, it relates to a joint between the railmember and respective rung members of said ladder.

Ladders are often used around high tension lines to position a workmanto reach certain components. Often the work is performed at aconsiderable elevation off the ground, thus making a relatively longladder mandatory. Metallic ladders are strong, but they are subject tothe disadvantage that they conduct electricity and thus increase thehazard to a lineman working from the ladder.

v Referring to FIG. 1, 11 is a ladder embodying the present invention.It has elongated rails 13 and 15 interconnected by a multiplicity oftubular rungs 21.

Referring now to FIGS. 24, it will be seen that rail 13 has an outercasing or sleeve 17 and a central core 19. The casing is preferably madeof a reinforced plastic, mg, a glass fiber reinforced epoxy, and thecore is preferably a unicellular plastic foam that is light andsubstantially impermeable to moisture, e.g., polyurethane. For asuperior dielectric pole, the casing and the core should be bondedtogether to form a hermetic seal there: between. My Patent 2,997,529discloses the preferred materials and method of manufacture for anelongated electrical insulating rod, which I prefer to utilize for rails13 and 15.

Rungs 21 are mad-e of a strong material, preferably a I dielectric.Various materials can be used, e.g., fiber glass a tendency to soak ,upmoisture; moreover, their mechanical strength for a given weight israther low.

It is therefore the object of the present invention to provide a ladderwhich will overcome the disadvantages of metal and wood in the priorart; more specifically, it is an object to provide such a ladder made ofreinforced plastic with a moisture-resistant unicellular core and havingrungs of a suitable dielectric material; it is a further object toprovide simple and eflicient joints between the rails and rungs on. saidladder; and yet another object is to provide a method for making such aladder and particularly for forming the joints for same.

In summary, the present invention concerns a ladder composed of spacedparallel rail members made of annular reinforced plastic casing orsleeve filled with a foam center and amultiplicity of dielectric rungsinterconnecting said rail members. The joints between the railsand rungsdepend on a bonded plastic collar which extends from the end of eachrung intoa socket-like interior in the rail. The collar is bonded tointernal. rail surfacesand it is ofa larger diameter than the mugreceiving opening which extends outwardly through the sides of thecasing to receive and engage the rung end. The method of forming thejoint contemplates that a hole will be drilled into the side of the railand that an enlarged cavity will be dug out in the foam interior incommunication with the exterior of the casing through the hole therein.The collar is formed in situ by filling the cavity with a curable liquidplastic material and curing said material to harden it while a rung issupported in place.

For a better understanding of the invention, reference may be made tothe accompanying drawings wherein:

FIGURE 1 illustrates in elevation a ladder made in accordance with thepresent invention;

FIG. 2 is a transverse section, taken along line 22 through FIG. 1;

FIG. 3 is a section taken along line 33 of FIG. 2;

FIG. 4 is a section taken along line 4-4 of FIG. 2; and

FIG. 5 is a transverse section illustrating the assembly of the rung tothe rail.

rein-forced polyester rod, but I prefer that rungs 21 be made of thesame materials as rails 13 and 15. Thus each rung 21 has a tubularportion 23 of glass reinforced plastic and an internal core 25 ofunicellular plastic foam. The end portion 27 of rung 21 extends into theinterior of rail 13 through the generally circular opening 29 in sleeve17. The extremity of end 27 abuts the internal surfaces of casing 17that lie opposite thereto, -as indicated at 31 and 33 in FIG. 2. Thefoam core 25 of rung 21 terminates short of the outer extremity of rungend 27, thus providing exposed internal surfaces 35 of the reinforcedplastic tubular portion 23.

Retainer collar 37 is bonded securely to the rung end 27 and extendsradially outwardly therefrom to such a point that the over-all diameterof end 27 and collar 37 is significantly greater than the diameter ofopening 29. This collar extends continuously from the inner wallsurfaces 39 of casing 17 lying adjacent opening 29 to the inner wallsurfaces 41 of casing 17 lying opposite opening 29, the casing beingbonded tightly to these surfaces. The outer lateral surfaces. 43 ofcollar 37 are configured generally as a right circular cylinder.

Foam core.19 encircles and jackets surfaces 43. Collar 37' extends intothe recessed interior in the extremity of rung end 27 and in bondedengagement with the exposed internal rung surfaces 35 as well as withthe'exposed end surface of foam core 25.

It will be evident from the foregoing that rung 21 is securely retainedfrom axial motion outwardly from rail 13 since collar 37 is larger thanopening 29 and cannot pass therethrough and because of the variouslydescribed bonded connections between the rung end and the internalsurfaces of rail casing 17. Moreover, the joint between rung 21 and rail13 is so constituted that transverse loads applied to rung 21, acting incantilever thereon, are resisted since the collar 37 integrally bondsthe rung to the internal wall surfaces 39 adjacent opening 29' as wellas to the opposite wall surfaces 41. Thus,viewing the rung as a beamloaded transverse to its axis, it effectively has two points of supportin its joint with rail 21: one at its end-most extremity, and the otherdisplaced therefrom along the axis of rung 21 approximately the internaldiameter of casing 17. 7 1

Although the collar .37 is bonded. securely with the jacketing foamsu'rfacesin rail 13, and with the circular end surfaces of foam core 25of rung 21, these joints cannot be depended on to ,add substantialstrength to the over-all joint because of the structurally weaknature ofthe unicellular foam, it being poor to resista load pin either tensionor compression. So, while the foam-plastic collar bonds are excellentfor sealing out moisture, they are of little assistance in adding jointstrength.

The above described joint is formed in a relatively simple manner.Referring to FIG. 5, hole 29 is drilled through casing 17 and on throughfoam center 19, terminating against the internal wall surfaces 41 of thecasing lying generally opposite hole 29. Then a suitable tool, forexample, a drill with an expandable reamer means, is manipulated toremove additional, foam and enlarge the passageway through the foam todefine enlarged cylindrical foam passage 51, which is of considerablygreater diameter than hole 29. A knife or other instrument is used toscrape the foam from against the internal wall 41 opposite opening 29and from the internal wall areas 39 just adjacent opening 29. Theloosened particles of foam are removed from the internal rail cavity byinverting the rail and tapping it lightly or by blowing air into theopening with an air hose.

A recess in the end of rung 21 is prepared by removing foam to exposeinner wall surfaces 35.

A measured quantity of liquid collar-forming material 53 is poured intocavity 51 while :the rail is positioned with the opening 29 upright sothat the collar forming liquid 53 will not run out. The collar-formingliquid is a polymerizable plastic resin that will bond quite tightlywith the exposed internal wall surfaces 39 and 41 of rail 13 and of theouter exposed surfaces of rung end 27 and with internal rung surfaces35. Likewise, it should form a good moistureseal bond with the foamcavity defining surfaces 51 and the circular foam end of rung core 25.The material should be quite strong yet it should not be brittle. I findthat amine-cured epoxy resin which has been modified by an additive tomake it somewhat more flexible, and thus more resistant to crumbling, isquite suitable. One acceptable resin may be obtained by mixing twocomponents commercially available from Shell Chemical Company under thedesignations, Shell Epon 828 and Shell X7l additive. Approximately 1:4ratio of additive to resin, by weight, is preferable. Anothercommercially available resin is Cibas Araldite No. 502. This resinrequires no further additive. Both of the above resins cure at roomtemperature, and, depending on temperature and other ambient conditions,curing takes twenty minutes to an hour after addition of 6% to 8% byweight of an amine-curing agent, for example, diethylenetriamine.

Promptly after the liquid resin and curing agent, which are mixed justbefore pouring, are introduced into the cavity, rung 21 is placed. intoposition by inserting rung end 27 into the cavity 51 and pushing itsend-most portion into contact with the opposing inner wall surfaces 41of rail casing 17. The positioned rung and rail are supported by asuitable ji-g until curing is substantially completed.

If an excess of liquid collar-forming material is used, it will beforced out through the slight clearance between hole 29 and rung 21 whenthe rung is inserted. This can be easily cleaned off by rubbing with amethyl ethyl ketone soaked rag.

Considering the assembly of a plurality of rungs, it will of course beappreciated that all the joints for at least one rail will have to beinserted substantially simultaneously. Since the resin takes severalminutes before substantial curing occurs, there is ample time to filleach of the cavities with the liquid resin and position all of therungs.

If desired, the ladder assembly may be accomplished by assembling therungs to the rails, with all rung ends in place, and, while supportingthe assembly by clamps or the like, introducing the collar-forming fluidinto each of the joint cavities. This may be accomplished by drilling va small hole through the tubing of the rails to communicate with each ofthe internal cavities and introducing the liquid therethrough. This hasthe advantage of per- Initting that a seal be made between rung 2-1 andrail 13 beforehand in order that resin may not run out therebetween torequire clean-up effort.

By way of example, a ladder of the instant invention may have side railsof approximately 2 /2 inches diameter the rungs being 1% inches indiameter. The diameter of the foam cavity in the rail at each jointwould run say A to of an inch greater than the diameter of the rungreceiving opening in the outer tubular portion of the rail, whichopening is just a bit larger than the diameter of a rung. For a 1%inches rung, the hole would be about 1.26 inches. The quantity of resinand curing agent required is approximately 50 grams when the wallthickness of the outer tubing of the 2 /2 inch rail is 0.155

1 inch, the Wall thickness of the 1%, inch rung 0.090, and

the depth of the end cavity in the 1 /4 inch rung /2 inch.

It will be appreciated that while my ladder is expected to find itsgreatest use as a safety ladder for work around high tensionelectricity, it will find some degree of application as ageneral-purpose ladder because of its light, strong construction.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

1. The method of joining an elongated tubular plastic sleeve filled witha foam core to a tubular rod-like member of smaller diameter than saidsleeve and also filled with a foam core, comprising:

(1) forming a hole through the side of said sleeve;

(2) forming a cavity communicating with said hole by removing foammaterial from the interior of said sleeve, said cavity depth beingdefined by rough internal wall surfaces of said sleeve adjacent as wellas generally opposite the hole in said sleeve, said cavity diameterbeing made greater than the diameter of said hole;

(3) forming a depression of substantial depth in an extremity of themember by removing foam material therefrom to produce a depression inthe member of substantial depth and exposing a portion of the roughinternal surface of the member of major area;

(4) introducing a sufiicient quantity of curable liquid plastic intosaid cavity and the depression to completely fill the same; and

(5) inserting said one end of said member through said hole and intosaid cavity and permitting it to remain so positioned while the plasticliquid material cures to form a solid mass bonded to said wall surfacesof the second member and to the internal surfaces of said sleeve. 2. Themethod of claim 1 wherein said hole is round and said cavity is formedcoaxially inwardly and generallyround in cross-section. Y

3. The method of claim 1 wherein is included the step of scraping thefoam material out of the sleeve and said one end of the member to assurethat the exposed interior surfaces of the sleeve and member are ofgenerally rough characteristics prior to step (4).

References Cited UNITED STATES PATENTS 1,558,498 10/1925 Peck et al264-2 62 X 1,742,625 1/ 1930 We-ckerle 264-262 X 2,040,126 5/ 1936Grieve 144-309 2,862,650 1 2/ 1958 Scott et a1. 182-46 3,009,532 11/1961Richard et a1. l82-4 6 3,042,140 7/ 1962 Basile et al 18246 3,272,894 9/1966 Roach 264-262 X EARL M. BERGERT, Primary Examiner.

H. ANSHER, Assistant Examiner.

1. THE METHOD OF JOINING AN ELONGATED TUBULAR PLASTIC SLEEVE FILLED WITHA FOAM CORE TO A TUBULAR ROD-LIKE MEMBER OF SMALLER DIAMETER THAN SAIDSLEEVE AND ALSO FILLED WITH A FOAM CORE, COMPRISING: (1) FORMING A HOLETHROUGH THE SIDE OF SAID SLEEVE; (2) FORMING A CAVITY COMMUNICATING WITHSAID HOLE BY REMOVING FOAM MATERIAL FROM THE INTERIOR OF SAID SLEEVE,SAID CAVITY DEPTH VEING DEFINED BY ROUGH INTERNAL WALL SURFACES OF SAIDSLEEVE ADJACENT AS WELL AS GENERALLY OPPOSITE THE HOLE IN SAID SLEEVE,SAID CAVITY DIAMETER BEING MADE GREATER THAN THE DIAMETER OF SAID HOLE;(3) FORMING A DEPRESSION OF SUBSTANTIAL DEPTH IN AN EXTERMITY OF THEMEMBER BY REMOVING FOAM MATERIAL THEREFROM TO PRODUCE A DEPRESSION INTHE MEMBER OF SUBSTANTIAL DEPTH AND EXPOSING A PORTION OF THE ROUGHINTERNAL SURFACE OF THE MEMBER OF MAJOR AREA; (4) INTRODUCING ASUFFICIENT QUANTITY OF CURABLE LIQUID PLASTIC INTO SAID CAVITY AND THEDEPRESSIION TO COMPLETELY FILL THE THE SAME; AND (5) INSERTING SAID ONEEND OF SAID MEMBER THROUGH SAID HOLE AND INTO SAID CAVITY AND PERMITTINGIT TO REMAIN SO POSITIONED WHILE THE PLASTIC LIQUID MATERIAL CURES TOFORM A SOLID MASSS BONDED TO SAID WALL SURFACES OF THE SECOND MEMBER ANDTO THE INTERNAL SURFACES OF SAID SLEEVE.